The invention relates to film capacitors, stacked or wound, in particular metallized wound capacitors. The method of impregnating the windings may also be applied to coil and other electrical components.
Film capacitors are to be understood as relating to capacitors made from paper and/or plastic material. This expression includes both metallized constructions as well as film/foil constructions. The foil here denotes a metal foil, e.g. Al, Sn, SnPb, etc.
For the purpose of this document the expression xe2x80x9ccapacitor windingxe2x80x9d also applies to xe2x80x9cstacked capacitorsxe2x80x9d, the expression impregnation relates to the impregnation of the winding as such, which is followed by an encapsulation. In the detailed description the invention will be described with reference to a wound film capacitor. This should, however, not be considered limiting the invention.
Wound capacitors are known within the art e.g. U.S. Pat. No. 4,656,556, EP, B1, 0 192 818, and DE, C1, 35 05 888. Such capacitors are e.g. wound from films that have been metallized. The metallization affords for the ends of the wound capacitor winding being metallized in order to form contacts on the capacitor. The films may be made from paper or plastic.
Paper has a drawback in that it is more porous and may have trap air in the pores. The usual procedure in manufacturing these capacitors is to dry the paper and thereafter impregnate it with e.g. epoxy in order to remove air from being trapped within the paper used in making the winding. Air may also be trapped within the winding itself between the different layers. Thus, when using plastic films instead of paper, air may still be trapped within the wound capacitor winding.
An often-felt need is to add self-extinguishing substances to the material forming the encapsulation of the capacitor. There are several substances like bromine, aluminum hydrate etc., which may be used for these purposes. However, some of these substances pose a problem when impregnating the capacitor winding since they hinder an effective impregnation to take place and substances like bromine is for environmental reason no good choice, thus they should not be used for the impregnating part of the manufacturing of the capacitor.
According to prior art paper capacitors are commonly prepared by winding paper, metallized or with thin metal foil, in two or more layers, into a winding. These windings may be provided with a vapor barrier at this stage by winding into the outer part of the winding a foil of suitable material. The windings, having a cylindrical form, are compressed into an essentially flat form and end-sprayed with or without leads or the like. It is important that the winding is sufficiently compressed such that no metal enters between the layers in the winding. The winding is dried thereafter and if the leads were not attached to the winding in the earlier step it is now soldered to the end-sprayed parts of the winding. The winding is mounted in a box or in a mould, and if no vapor barrier has been applied to the winding in the earlier stages the vapor barrier in the form of a label may be placed in the mould.
The mold or the box with the winding is subjected to vacuum and impregnation of the winding including filling the box or the mould is accomplished, using the same medium for impregnation and filling and doing this in one step. The box/mould and its contents are subjected to curing at this time and the finished capacitors are either pulled out of the mould or the boxes are separated. The capacitors are then of course tested and marked before packing the same.
In the case of windings being made from plastic material instead of paper, the filling of the boxes or moulds is done without applying a vacuum.
An overview of the manufacture of the capacitors according to the prior art is given in table I, column 1-3.
The invention concerns a new method for preparation of impregnated and encapsulated capacitors.
According to the invention the method of making the windings comprises a step in which it is ascertained that the air is removed from the finished winding and that the impregnating material is cured.
According to the invention it is also possible to impregnate the winding using one impregnating medium and thereafter encapsulating the same using another medium.
The winding is wound from metallised film, paper or plastic. The plastic film may e.g. comprise polypropen (PP), polyester (PET), polycarbonate (PC), polyphenylene sulphide (PPS), polyethylene naphtalate (PEN). The choice of film material is dependent on the intended use for the capacitor.
In the new method according to the invention a winding/stack is prepared. The winding is preferably placed under vacuum in order to remove the air and to impregnate the winding with e.g. epoxy, the vacuum is thereafter released and the air entering exerts a pressure on the epoxy, which thus is forced into the capacitor windings. The winding with the impregnating epoxy is cured in a curing bath and leads are fastened to the capacitor windings in order to provide for electrical connections for the capacitor. Thereafter the capacitor winding is preferably placed into box for the encapsulation with e.g. epoxy, whereafter the epoxy is cured. In this manner the impregnating agent used for the impregnation of the winding may be different from the material used for the encapsulation of the same.
It is an object of the present invention to provide an encapsulated, wound capacitor, paper or plastic, which can be manufactured in a simple and efficient manner.
It is further an object of the present invention to provide an encapsulated, wound capacitor, paper or plastic, which has an encapsulation, which provides self-extinguishing characteristics and shows strength and resistance towards common fluxing agents, and where the impregnation of the winding can be made using compounds more suitable from environmental reasons, than the compounds used hitherto.
It is also an object of the present invention to provide a winding for a capacitor, paper or plastic, which is easily and effectively impregnated and cured and where the impregnating agent is more acceptable from environmental reasons, than the compounds used hitherto.
It is further an object of the present invention to provide an encapsulated, wound capacitor, paper or plastic, in which the wound capacitor winding exhibits high voltage withstand capability, i.e. there should be no risk for short-circuits.
Metallized paper film provides one good characteristic over plastic film and that is the superior ability of self-healing not provided for by plastic films. Plastic film, however, film metallized with very thin metal layer exhibiting a high xcexa9/xe2x96xa1 gives almost equivalent results in this respect.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.